Protective device.



E. E. F. CREIGHTON.

PROTECTIVE DEVICE.

APPLICATION HLED MAR.19,1913.

Patented June 22, 1915.

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E. E. F. CREIGHTON.

PROTECTIVE DEVICE.

APPLICATION HLED MAR.19, 1913.

Patented June 22, 1915.

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UNITED sTA'rns 'PATENT OFFICE.

ELMER E. F. CiBiEIGH'ION, 0F SCHENECTADY, NEW YORK, ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION NEW YORK.

PROTECTIVE DEVICE;

7 'o all 'w11 om, t may concern Be it known that I, ELMER E. F.Cnmen'roN, a citizen of the United States, residing at Schenectady,inthe countyof Schenectady, State of New York, have invented certain newand useful Improvements in .Protective Devices, of whichrthe followingis a specication.

My invention relates to devices for protccting circuits from. abnormalvoltages, lightning and similar disturbances, andl more especially toprotective devices for circuits of low normal operating voltage.

The protective device most commonly used for telephone circuits, signalcircuitsand similar systems of low normal operating voltage is a form ofspark gap lightning arrester comprising essentially two electrodesseparated by a very short air gap. The air gap has an appreciable timelag, as it will not break down the very instant an excess voltageappears, and is furthermore liable to be bridged by dust, dirt and bythe metal carried from the electrodes by a heavy discharge.-

The object of my invention is to provide a protective device which doesnot have the defects of the usual spark gap lightning arrester and whichwill remove abnormal voltages and transient-disturbances withoutdangerousdelay and before the abnormal voltage rises to dangerous value.

My invention comprises certain novel features set forth moreparticularly in the appended claims, but the invention itself will bebetter understood in connection with the accompanying drawings, whichmerely for purposes of illustration show some of the various forms inwhich my invention may be embodied; and in Which- Figure 1 is a planview of one form of a lightning arrester embodying my invention; Fig. 2is a longitudinal section of the form of arrester shown in Fig. 1 withthe electrodes and some of the interior parts shown inelevation; Fig. 3is a longitudinal section of an arrester shown in Fig.- 2 in which lossof the vacuum in the devicey short circuits the electrodes and permitsthe defective arrester to be detected-by simple tests; Fig. 4 is alongitudinal section of a modification, in which a desirable form ofseal is shown and the electrodes are so arranged that the arcing takesplace in a sort of circular horn I Specification of Letters Patent.

Application led March 19, 1918. Serial No. 755,529.

gap, the electrodes also having fusible disks on them; Fig. 5 shows atransverse section of the form of arrester shown in Fig. 4 on the line5-5; Fig. G is a longitudinal -section of a modification in which thedischarge occurs between electrodes which are far removed from the sealand are so mounted that the heatof discharge does not aiiect the seal;.Fig 7 is a longitudinal section of another form of arrester iii whichthe discharge takes place at such a distance from the seal that the heatof the discharge does not affect the seal; and Fig. 8 is a longitudinalsection of a' form of arrester in Which the. discharge takes place in ahorn gap mounted in the vacuum chamber.

In the particular form of device shown in Fig. 1 the ari-ester ismounted upon a porcelain base 1 provided at each end withA terminals 2by means of which connection is made to the conductors of the circuit tobe protected. A ground connection or terminal 3 mountcdncar the middleof the porcelain base 1 engages and holds in position a metal tube 4which is closed at each end in any suitable way to forni a vacuumchamber which contains the electrodes of the spark gap. By suitableconnections the line electrodes mounted in the tube are connected to theterminals 2 and when an abnormal voltage or other disturbance appearsupon either conductor of the circuit a discharge will occur from thecorresponding electrode to the metal tube 4, which forms the groundelectrode. The metal tube is exhausted to that degree of vacuum which ismost effective in permitting discharge of abnormal voltage to ground andpreventing a flow of current at the normal voltage. By making the vacuumchamber of metal I am able to dispense with a special ground electrode,lmaintain the vacuum more nearly constant during rdischarge on account ofthe heat radiating capacity of the metal tube 4, materially reduce thesize of the vacuum chamber, as the heat of discharge will not crack orinjure the metal tube 4, and in general produce a much more rugged anddesirable device than would be possible with a vacuum chamber of glass.

In order to prevent the arrester being partially disabled by dust anddirt collecting between the terminals 2 and thc tube 4 and forming aconducting bridge between them I provide means for protecting the endsof Y between the terminals, but the preferred constructionl is 'thatshown in the drawing, 1n

V 1.0 which each end of the tube is inclosed in an,

connections can be established between the terminals and-the tube exceptthrough the -electrodesinside of the tube. Various forms of constructionmay be adopted to protect theend of the tube and shield it from theinsulating cap 5, preferabl made of porcelain and extending along t eexterior of the of metal plates set in t tube 4 `to such a 'distancethat the tube is thoroughly protected from contact with the terminal 2."f

The electrodes inside ofthe tube 4 are connected to the terminals 2through end connections 6 preferably made in the form e insulating caps5 concentric with the axis of the tube 4. These metal plates areconnected t'o the outer end of heavy metal leadin in wires 7 whichextend longitudinally o the tube and carry on their inner endselectrodes 8 preferably made in the form of disks mounted concentricwith the tube 4. The electrodes 8 are spaced -away from the walls ofthemetal tube 4to form a spark gap through which the normal voltage of theline cannot maintain a discharge, even at the reduced pressure oftheresidual gases in the' tube 4. A dangerous voltage, particularly 1ftransient or oscillating, will break down the dielectric between theelectrodes and the walls of the tube 4 and `discharge through the tube 4and the terminal 3 to ground.

In order to prevent spattering of molten metal from one electrode to theother in case of a heavy discharge from one line to ground and ingeneral to protect the electrodes from each other, I provide betweenthem a porcelain spacer 9 preferably made in the form shown in Fig. 2and of such a size that it offers suiiicient cooling surface to preventmolten metal from one electrode spattering to the other electrode andshort circuiting the arrester by establishing a conductingbridge'between the other electrode and the tube, or between the twoelectrodes. The electrodes 8 are supported and positioned in the tube inanysuitable'way,

preferably by means of porcelain or other' insulating supports 10, whichmay be made in various forms, but are preferably made as shown in thedrawings in the form of disks fitting into the tube and having a centralprojection or hub through which the leading-in wires extend.

Since under some vconditions a heavy discharge current may flow throughthe leading-in wires 7 and :the electrodes 8, some form of seal must beused which will .not leak when subjected to variations in temperatureand which .will not give oif gases and thereby deteriorate the vacuum ofthe tube dustand dirt-which might form a bridgevable vitreous or aurons4 if the discharge is' heavy enough to heat the leading-in wires. Iprovide such a seal by filling the tube adjacent the electrode support10 with a hard insulating material, which need not-be air-tight, butwhich will not give 0H any gases .when heated and which. can be fixedinY osition in the tube. For this purpose I pre er to use somesuitorcelain like material made in the form o a stopper 11 and placedimmediately lbehind the electrode support 10. This stopr will not giveoif ases when heated, s elds the remainder o the seal from the heat ofthe arc and acts as a support to prevent displacement of the vis. cousmaterial forming the remainder of the seal. The stopper 11 is backed upand rendered air-tight by a vfilling of soft wax 12.

which is impervious to air and always makes an air-tightj'oint, `bothwiththe wallsof the |tube and with the stopper 11, thereby sealing upall of the'pores 1n the stopper 11 and preventing the entrance of airinto the tube 4. This layer of wax 12 preferably consists of pitch orother soft material which will 'adhere firmly to the tube and also tothe stopper 11. This layer 12 is in turn backed up by another layer 13of a wax which is more firm and solid than the soft pitch 12 and whichalso makes a tight joint with the walls of the tube and thelead.-ing-'in wire 7. A seal constructed as just described is not opened ordestroyed by variaf tions in temperature either of the tube or of s theleading-in wire 7, as the soft wax 12 and the firmer wax 13 will alwaysmake an airtight joint with the tube and leading-in wire under allconditions of temperature to which l the device will ordinarily besubjected. The stopper 11 is strong enough mechanically to reventdistortion of the wax layers 12 an 13 by pressure, and as a result Iobtain a seal which will not leak' under any conditions to which thearrester will ordinarily be subjected.

The protective device constructed as above described is exhausted toproduce in the metal tube a partial vacuum of such a value thatlightning and similar abnormal disturbances are removed from the linewith minimum delay and resistance. The desired vacuum may easily beobtained by pumps, such as are commonly used in the art, and after thetube is exhausted it may be hermetically sealed in any suitable way. Inthe opening or connecting the tube to the pump is sealedby means of adrop of solder 14 when the vacuum in the tube is of the proper value. Insome cases it may be preferable to exhaust the metal tube 4 through asmall tube made of solder which can then be melted and sealed oit' inthe same manner as the glass tubulature of anincandescent lamp.

In many cases I prefer to first fill the tube with some suitable gas andthen exhaust it to the required vacuum, so as to leave in the tube avresidual atmosphere of that gas. I prefer to use as a residual'gas someof the monatomic gases, such as argon, neon, or helium, especially theinert atmospheric gases, such as argon or neon, although a residualatmosphere of hydrogen air or nitrogen will give satisfactory results'.The proper pressure of the residual gas depends upon the nature of thegas in the tube and vmined by connecting in parallel between the leadsof the same circuit the protective device and an outside spark gapformed by needles which are. adjustable with relation to each other.Sufficient high frequency Ipotential is applied to the circuit to causeoscillating disruptive discharges through either the protective deviceor the outside spark gap, or both. In practice I prefer to start withthe needles far apart so that all the discharges go through the arresterand gradually bring them together until a majority of discharges passthrough the needle gap. This test, while not the best way of determiningabsolutely the equivalent needle gap, is rapid, gives consistentresults, and is accurate enough, especially for purposes of comparison.To secure the desired result the frequency of the applied potential mustbe of the order of that of the spark discharge of a condenser, and inpractice I obtain the desired high frequency potential by a condenserdischarging through gaps. The `frequencies obtainable by any mechanicalor similar interrupter are too low to be of value for this purpose. Thatvalue of outside needle gap in parallel, which just causes theprotective device to take the majority of the discharges, is vtheequivalent needle gap referred to in connection withthe test abovedescribed: With the gas in the protective device at atmosphericpressure, this equivalent needle gap is relatively high. As theprotective device is 'exhausted the equivalent needle gap diminishes andpasses througha minimum `known as the critical pressure of the spark gapwhen the pressure of a residual atmosphere of hydro` gen in theprotective device is between 8 to 18 mm.' of mercury. The pressure canbe considerably higher than this Without causing an objectionableincrease in equivalent i needle gap, but further exhaustion beyondcritical pressure lcauses the equivalent needle gap to increase veryrapidly. This pressure seems to be substantially-constant andsubstantially independent ofthe length of the spark gap when the gapisbetween about one-half mm. and one cm. in length. In an arresterembodying my invention I prefer to maintain the pressure of the residualgas within this pressure range or a little greater than the criticalpressure of the spark gap over which the arrester has the minimumequivalent needle gap.

A disruptive spark discharge furnishes favorable path for di'scharges toground and is the kind of a discharge most suitable for relieving asystem of lightning, highfrequency oscillations and similar abnormalpotentlals and frequencies. A brush discharge carries but little currentand does not relieve the line of very much energy. Byheating up theelectrodes the brush discharge may actually hinder the formation of thespark discharge and the relief of the system, especially where thearrester is used on comparatively low voltage vdirect current systems. Iso space the electrodes that at the preferred pressure the spark dis-`charge occurs almost simultaneously with the appearance of a brushdischarge and 4at a voltage somewhat above the normal voltage of thesystem to be protected. This preferred pressure is above the criticalpressure at which the spark potential is a minimum. With brasselectrodes, which I 'preferto use, the proper length of gap is about 1mm. which at the pressure of minimum -equivalent needle gap gives anarrester which breaks down with practically no brush discharge and'whichhas a spark potential very slightly above the minimum spark potential.

A lightning arrester should permit the passage of transientover-voltages and very high frequency oscillations with a minimum timelag. By high frequency I mean oscillations and oscillatory discharges ofthe frequency of lightning or of the spark discharge of condensers,probably from vonehalf million to several million cycles per second. Thetime lag must be so small that the transient over-voltage will not have:

time to puncture the insulation. The'spark discharge should occurwithout appreciable time lag in responseto the application of atransient high voltage -of minimum amount over the normal potential ofthe system. I reduce the time lag to an unobjectionable amount byadjusting the gas pressure and the .spark gap without seri* ouslyaffecting the minimum equivalent needle gap. I have found that atpressure above 10 mm. of mercury the time lag is not enough to beobjectionable, but that it is serious at lower pressures, increasingvery rapidly as the pressure decreases. I, therefore, make the pressuresuch and so space the electrodes that the time lag is unobjecnear theminimum, and spark potential is reasonably low.`

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Through various causes the vacuum in the protective device may-be lost,1n whlch case it is desirable that some wa be provided for -detectin thedefective evice. In accordance wit my invention I provide means wherebythe electrodes 8 are automatically bridged or short circuited as soon asthe vacuum is lost, whereupon the defective arrester may easily belocated by simple tests. The electrodes 8 may be short circuited 1nvarious ways but the construction which I prefer is that shown in Fig.3, 1n whlch a short circuiting or bridglng member 15 1s pivotallymounted upon a sgmcer 16 corresponding to the spacer 9 o F1g.'2 and 1sautomatically swung into short cu'cultmg position whenl the vacuum islost by some suitable means which responds to the 1ncrease in pressurein the tube 4. A sultable device for swingin the short circuitmg member15 is a resi ient diaphragm .17, as shown in Fig. 3, and normally held1n the position shown by the pressure of the atmosphere but biased tomove outward and swing the bridging member into engagement withelectrodes 8 as soon as the pressure 1n the tube is increased by loss ofvacuum.

The tendency to extinguish the are between the electrodes and the tubemay be increased by -positioning theelectro'des eccentrically withrelation t`o the tube so that the are occurs in a sort of circular horngap in which it is automatically lengthened and ultimately extinguished.One form of arrester in which the extinguishing effect of the horn gapis secured is illustrated in Fig. 4, in which the tube 18 containselectrodes 19'n1ounted eccentrically to a ground ring 20,.which asbestshown in Fig. 5, is electrically connected between the electrodes toground. The electrodes 19 and the ground ring 20 are carried in a yoke21-made of orcelain or other` suitable insulating material'and ittingthe tube 18 so as to Ahold the parts in proper relation. Stoppers 22 aremounted adjacent the yoke 21 and are made of some suitable porcelainlike material which can be baked in the tube and which will not give oifgases when heated. The Stoppers 22 are backed up by layers 23, 24 and 25of diii'erent kinds of waxes which form air-tight joints with the tube18 and also with the leadin -in wires 26, which are connected at theirinner ends to the elec# trodes 19 and at theirouter ends to connectionplates 27 imbedded` in the porcelain end blocks 28..

The vacuum used in the form of arrester shown in Figs. 4 and 5 ispreferably the same as that used in arresters of the type shown in Fig.2. Under some conditions I accentuate the arc extinguishing feature ofthe circular horn gap formed between the eccen trically disposeelectrodes 19 and the ground ring 20 by so proportioning the spacearound the electrodes that upon a severe and long continued discharge te residual gases in the arrester'expand to such an extent that theirpressure is very greatly increased and their arc extinguishing effect iscorrespondingly increased. The arc extinguishing eiect of thearrestersshown in the other res of the drawin may be increased in t esame way if consldered advisable, although I prefer to so proportion thearrester shown in Fig. 2 that the pressure of the residual gases 1smcreased but slightly during the usual discharge.

Discharges through the protective devicev grounded to protect theapparatus connected to it. In addition the continuous and heavydischarges heat the electrodes and the adjacent portions of the arresterso hot that sufficlent gases are evolved to lower the vacuum,

and in general the arrester is deteriorated to such an extent that itshould be removed and replaced by a new one. In order to automaticallyground the system when crossed wlth a hlgher potential system, and tofacilltate detection and removal of the arresters which have beeninjured, I provide means whereby the electrodes are automatlcallybridged and short circuited and are also connected to ground when theelectrodes are heated to a certain extent. The preferred means forsecuring this result, as apphedto'the particular form of arrester shown1n Fig. 4, comprises fusible metal films 29 secured tothe faces of theelectrodes 19 and made in `the form of disksof less diameter than theelectrodes. The arc or discharge from the edge of 'the electrode 19Idoes not touch the fusible disk, which is heated only by conduction ofheat from the electrode. As the disks 29 are in intimate contact withthe electrodes 19 a rise in temperature of the electrodes will cause thedisks 29 to melt and the molten metal will run down and form aconducting bridge between both electrodes and the ground electrode,which in this case is the ground ring 20. Whenvthis has occurred theshort circuited arrester may easily be detected by suitable tests andthen replaced.l The fusible disks 29 may obviously be applied to theelectrodes 8 of the arrester shown inFig. 1, if desired, inwhich casethe molten metal connects both electrodes to the grounded tube 4.

In some forms of arresters designed to withstand severe and continuedldischarges 1,144,028 y vw it is desirable to protect as far as possiblethe waxes .and otherv lmaterial of the seal from undue heating by theaction `of the arc between the electrodes. I attain this result i byplacing the electrodes at a considerable distance from the sealy andconnecting them to the leading-in wires through metallic connections soproportioned as to have a very Vthe discharge. 4In this arrester thetube 30,

exhausted'to a suitable degree of vacuum, is

provided with two electrodes 31, one connected to line and the other toground. Each electrode is mounted on a radiator 32 which exposes a veryconsiderable surface to the residual gases in thetube and is also quiteclose to the tube so that as the radiator becomes hot the heat isquickly reinovedcfrom it. Each radiator 32 is connccted to a leading-inWire 33 which extends through a stopper 34 of suitable refractorymaterial. Each stopper is backed up by a seal comprising a layer ofsuitable wax 35 contained in chambers formed around the leading-in wiresby suitable constrictions 36 in the tube 30, the wax being held in placein the constrictions by the Stoppers 34. The leading-in wires may'beponnected to line and to ground by terminals 37 on the ends of thetube V30. The radiators 32 hold the electrodes 31 as far as possiblefrom the seals and no heat can be transmitted Jfrom the electrodes tothe leading-in wires of the seal except by passing through the radiators32. Another device for securing the same result is shown in Fig. 7, inwhich an evacuated glass bulb 38 contains an electrode 39 mounted todischarge to an electrode 40 at a point which is as far as possible fromthe ends ,of the electrode 40 which are embedded in the wax-41 of theseal. In this form of arrester the discharge takes place as far aspossible from the seal and practically none of ythe heat generated atthe electrodes reaches the seal because of the great distance throughwhich it must-be transmitted and the amount of radiating surface of theelectrodes 39 and 40.

A modification in which the extinguishing action of the horn gaps issecured is shown in Fig. 8. An evacuated glass bulb 42 contains a groundelectrode 43 which coperates with two line electrodes 4-4 so positionedwith relation to the ground electrode 43 as to form therewith horn gapsin which the 55 arc may rise vertically and be extinguished.

To prevent the possibility .of'arcs following the electrodes 44down tothe seal and there-l by injuring the seal,'I coat both of the electrodesnear the seal with' a protective coating 45 of refractory insulating`material soA that the arc cannot reach the electrodes at points closeenough to-the seal to injure it. Iii this form of arrester theelectrodes are sealed `into the glass stein of the vessel 42, but theseal between the electrodes and the stem is not vrelied uporrto hold thevacuum.

The seal is made tighty by filling the stem with suitable wax 46..v

My invention may 'be embodied in many other ways than those describedand I, theref ore, do not limit my invention to the precise arrangementsdisclosed-except in so far 'as it is limited by the scope of theappended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is 1. A protective device comprising a chamber and spark gapterminals spaced apart within said chamber and surrounded by aluid at apressure which renders the equivalent needle gap of said spark gap aminimum.

2. A protective device comprising a chamberand spark gapterminalstherein, Said chamber containing fluid 'at a pressure which renders theequivalent needle gap of said spark gap a minimum, and said terminalsbeing spaced to have a low spark potential at the pressure existing insaid chamber. v

3. A protective device comprising spark gap terminals, and ay partiallyevacuated chamber inclosing said terminals and containing a monatomicgas at a pressure between 8 min. to 20 mm. of mercury.

4. A protective device .comprising spark gap terminals, and a` partiallyevacuated chamber inclosiiig said terminals and containingV gas at a.pressureggreater than the critical pressure of saidv gap but low enoughto render the equivalent needle gap substantially a minimum'. g

5. A protective device comprising spark gap terminals separatedby a gapof about 1 mm. and a chamber inclosing said spark gap and containingAgas at a pressure of 8 to 2O mm. of mercury.

6. A protective device comprising a sealed metal chamber containing aninert gas below atmospheric pressure, and an electrode in said chamberspaced away from the walls of said chamber to form a spark gap fordischarge of abnormal voltages.

7. A protective dev-ice comprising .an evacuated chamber having a'metalwall connected to ground, and twodisk shaped electrodes mounted face toface and concentric with said tube to form spark gaps therebetween, thedistance between the edges of said electrodes and said metal wall beingspaced away from the walls of said tube" o..

form spark gaps, and an insulating barrier between said electrodes,

9. A protective device comprising an evacuated chamber having a metallicwall, v

two electrodes mounted in said chamber and spaced away fromsaid wallt'o'form spark gaps therewith, and an insulating barrier. between saidelectrodes proportioned to prevent the formation of la conducting bridgebetween said electrodes:

10. A protective device comprising an evacuated chamber in the form of ametal tube closed at each end, disk electrodes mounted concentric withsaid tube and spaced away from the tube to, form spark gaps. and aninsulating cylindrical spacer loosely positioned in said tube betweensaid electrodes to prevent theformation of a conducting bridge betweensaid electrodes.

11. A protective device comprising an evacuated chamber in the form of ametal tube, disk electrodes mounted concentric with said tube, and aporcelain spacer between said electrodes having a cylindrical middleportion fitting loosely in said tube and smaller portions which projectfrom the ends of said spacer. y

12. In a protective device, the combination of an evacuated vessel andspark gap electrodes mounted in said vessel, of means responsive tovariations in the vacuum in said vessel for varying the resistance tothe passage of current between said electrodes.

13. A protective device comprising an evacuated vessel, spark I gapelectrodes mounted in said vessel, and means actuated byan increase inthe pressure in said vessel to establish a conducting path between saidelectrodes.

14. A protective device comprising an evacuated vessel, spark gapelectrodes mounted in said vessel, a metallic bridging member normallyout of electrical contact with said electrodes, and means responsive toan increase in pressure in said vessel for bringing said metallicbridging member into connection with said electrodes.

15. A protective device comprising an.

evacuated vessel, spark gap electrodes mounted in said vessel, abridging member for engagingsaid electrodes and normally out of contactwith said electrodes', and means actua-tedby an increase in pressure 1nsaid vessel for bringing said bridging member intol engagement with saidelectrodes and thereby short-circuiting the de- 16. In a protectivedevice, the combnation of an evacuated vessel, spark gapelectrodesmounted in said vessel, and leadingin wires which extend through theWalls of `said vessel, of a seal comprising a refractory stopperadjacent each electrode for shieldf 17. In a protective device,

trodes mounted in said vessel, and leadingin wires which extend throughthe walls of l 15 the combination of an evacuated vessel, spark gapelecsaid vesselof aV seal comprising refractory Stoppers of porcelainlike material mounted adjacent each electrode for shielding said sealfrom the heat of arc discharge', and a layer of soft impervious adherentwax to the walls of said vessel and to the leading-in 85 wires andimpervious to air immediately be' hind and in contact with said stopperand adhering to said stopper.

18. In a protective device, the combination of an evacuated vessel,spark gas elec- .j

trodes mounted in said vessel, and leadingin wires which extend throughthe walls of said vessel, of a seal comprising a ,plurality of layers ofwax-like material impervious sel and to said leading-in wires, the waxin said layers being of different degrees of hardness.

19. In a. protective device the combination of an evacuated vessel,spark gap ,100

'to air and adherent to the walls of said ves- 95 terial adjacent theelectrode, a layer of soft impervious adherent wax immediately adjacentsaid stopper and in contactvwith the Walls of said vessel and with saidleading-in wires, a second layer of adherent hard wax adjacent the firstlayer, and a solid insulat` 11 ing end piece engaging said second layerof lwax and the walls of said passage to form a closure for the openinginto said vessel.

20. In a protective device the combination of a metal tube, spark gapelectrodes mounted in said tube, and leading-in wires which extendlongitudialy of said tube, of a seal comprising a solid stopper ofrefractory in-` sulating material adjacent each electrode, a pluralityof layers of 'adherent wax impervious to air behind said vstopper andengaging the walls'of the tube and the lead-iY ing-in wires, saidVlayers being of different density, and an end piece of solid insulatingmaterial having a central opening for the passage of the leading-in wireand fittingA into the end of the tube to engage the wax mounted in saidvessel, and leading-in Wires a leading-in wire which projects from 45which extend intosaid tube from the ends, of a sealV for saidleading-inwires comprising a stopper of solid refractory insulating` materialadjacent each electrode, a porcelain end` piece -for each end of thetube having a central opening for the leading-in wire .and a viscousadherent wax filling for the tube between said lstopper and said endpiece.. n

22. A protective device comprising an evacuated chamberhaving metalwalls, an electrode mounted in said chamber, a ground terminal on themetal wall of' said chamber, a line terminal mounted adjacent the Wallof said chamber and connected to said j' electrode, and an insulatingshield substanlsaid chamber to form an insulating Wall between saidterminal and said chamber.

24. A protective device comprising an evacuated cylindrical chamberhaving metal walls, an electrode in said chamber having aleading-in'wire projecting from the end of said chamber, a terminalmounted outside said chamber near the end thereof and con-A nected tosaid leading-in wire, and an insulating cap for the end of said chamber,

- said ycap having a central opening for said leading-in wire andextending along the outside of said chamber from the end of said chamberto a point beyond said -termi-l nal.

25. A protective devicel comprising an evacuated ,cylindrical chamberhaving metal walls, an electrode in said chamber having ends of saidchamber, a porcelain end cap having side walls which surround saidchamber for a considerable distance from the end and a central openingfor said leading-in wire, an end connection secured to said leading-inwire and recessed in said porcelain cap, and a terminal provided with anextension for engaging' said connection and mounted adjacent saidporcelain'end cap to bring the side walls of said porcelain cap betweensaid terminal and the metal Walls of said tube.A

26. A protective device comprising an inisulatingv base having a recesstherein, an

evacuated metal chamber fitted in lsaid recess, a ground connection forsaid chamber near the middle thereof, electrodes 1n said chamber havingleading-in wires which project from the ends of said chambers,insulating end caps having side walls which inclose a considerable`portion of the end Walls of saidchamber and are provided with a centralopening through which said leadingin wires project, and terminalselectrically connected to said leading-in wires and mounted on said baseadjacent the side walls of said insulating caps.

27. A protective device `comprising an evacuated metallic tube closed atthe ends, electrodes inside of said tube, terminals mounted adjacenteach end of said tube and electrically connected to said electrodes, and

.insulating caps encircling the ends of said tube adjacent saidterminals and extending along said tube beyondsaid terminals to preventelectrical connection between said terminals and saidtube.

In witness whereof, I have hereunto set my hand this 17th day of March1913.

` ELMER E. F. CREIGHTON. Witnesses:

BENJAMIN B. HULL,

HELEN ORFoRD.

